It is well known to use security tags on retail items which trigger an alarm when the item is taken passed alarm units located at the exit of a store without prior removal of the security tag. These are known as Electronic Article Surveillance (EAS) labels or tags. There are 3 main types of devices: electromagnetic devices, acoustic magnetic (AM) devices (which are typically in the form of a flat strip or label approximately 45 mm long×40 mm wide) and Radio Frequency (RF) tags which typically comprise either a flat coil or wire approximately 50 mm in diameter or a coil wound around a ferrite rod to form an antenna. The tag acts as a transponder, i.e., it is energized on receipt of radiation, e.g., from an alarm unit, and then transmits a signal back automatically, e.g., to the alarm unit. Other types of tags are also known, e.g., Radio Frequency ID (RFID) tags, also known as “intelligent tags” as they send additional information with the signal they transmit. These types of tags and transponders are well known, so they will not be described further.
A problem that arises with some of these devices, particularly with electromagnetic devices such as RF tags, is that they can be shielded by a metal layer, e.g. the metal layer within a CD or DVD, yet it is desirable to be able to use this form of tag as a significant proportion of retail outlets use RF alarm systems rather than AM alarm systems. Such shielding may attenuate electromagnetic signals passing therethrough and/or cause frequency shift of the signals. RF tags comprising flat coils can be used but as they are relatively large it is difficult to mount them in a position when used with a container such as a CD or DVD box where they do not suffer from shielding.
A preferred embodiment of the invention aims to provide an arrangement of a security device which helps avoid or reduce this problem.